This is the first time in the F-35 Lightning II program that estimators have lowered the projected cost of the Pentagon's most expensive acquisition program. The annual Pentagon selected acquisitions report, which was published Thursday, is currently projecting the cost to develop and procure the F-35 Lightning II at a little more than $391 billion.

The F-35 Lightning II is one of 78 acquisition programs that was covered in the selected acquisitions report. It's worth noting that the F-35 Lightning II wasn't the only program able to stay under the spending cap covered in the program. The Pentagon undersecretary for acquisition, technology, and logistics, Frank Kendall noted that this was the "first time in my memory" that no program covered in the SAR went over the federal spending cap.

The F-35 Lightning II is broken down into two sub programs by the report including Lockheed Martin's portion of building the actual aircraft and Pratt & Whitney, the engine builder. $4.9 billion in savings actually came from Lockheed Martin, while Pratt & Whitney actually saw a cost increase of roughly $442 million due to development and procurement of the engines.

Currently, total flyaway costs the F-35A are $76.8 million with the Navy carrier-based version costing $88.7 million. The Marine Corps version, the F-35B, saw costs decline the most to $103.6 million per unit.

The entire F-35 Lightning II fleet had been grounded earlier this year when a crack was discovered in an engine component on one fighter. The aircraft were cleared to fly again in March.